The present invention relates to a relative positioning GPS (Global Positioning System) using a GPS surveying system and relates, in particular, to a relative positioning GPS in which, even if an interruption of receiving of radio waves from GPS satellites occurs, an integrated carrier phase (i.e., an integrated value of the carrier phase of the radio waves; also called a carrier beat phase) with a corrected "cycle slip" is recorded, and coordinate values of the three-dimensional coordinates of an unknown point can be obtained. The present invention also relates to an apparatus for carrying out a relative positioning GPS. In the present specification and claims of the application, the term "relative positioning GPS" means a method of positioning using the GPS surveying system. This method is also alternatively called a "GPS interferometry."
2. Description of Related Art
GPS satellites travel in circular orbits about 20,200 km above the earth's surface such that they rotate at a period of exactly two times per one rotation of the earth in the space. It follows that, when space is looked at from one fixed point on the earth's surface, the GPS satellites which travel spatially on the same orbits while gaining about 4 minutes per day can be observed.
On these GPS satellites there are mounted atomic clocks of secium and lubidium and they are used as a frequency standard in generating a reference frequency, e.g., for carrier waves for the radio waves or the like.
On the other hand, there is also provided GPS-related equipment on the ground, and an operation is being carried out to maintain the function of the GPS. For example, in order to grasp the accurate positions of the GPS satellites, there are provided on the ground satellite tracking stations for tracking the orbits of the GPS satellites over the entire surface of the earth. Measured results as observed in these satellite tracking stations are sent to a GPS main analyzing station for carrying out a concentrated analysis therein, thereby calculating expected orbits of the GPS satellites in the near future. The information relating to the expected orbits is transmitted to the GPS satellites and is transmitted from the GPS satellites to GPS users on the ground together with various kinds of information.
GPS satellites transmit radio waves of the following two kinds of frequencies with the frequency of 10.23 MHz as a basic frequency, i.e., one being L1 band of the frequency 1575.42 MHz which is 154 times the basic frequency and the other being L2 band of 1227.60 MHz which is 120 times the basic frequency. The L1 band contains a P code, a C/A code and navigation messages in the form of phase modulation.
Among them is included clock information. If the clocks provided in the GPS satellites and the clock in the receiving apparatus which is positioned in a survey point are synchronized with each other, the propagation delay time of the radio waves can be obtained from the difference between the time at which they are transmitted and the real time at which they are received. From this delay time and the speed of light the distance between the survey point and each GPS satellite can be calculated. On the other hand, since the values of the three-dimensional coordinates of each GPS satellite can be obtained from the orbit information, if there are three or more spheres each of which has a radius equivalent to the distance between each GPS satellite and the survey point, with the position of each GPS satellite being as an origin, the three-dimensional coordinates of the survey point can be obtained as a point at which these spheres cross each other.
This kind of positioning method is called a single or independent positioning method. The three-dimensional coordinate values obtained by this method are nothing but approximate ones and contain an error of several meters or more. They are therefore not suitable for high-accuracy positioning.
As a solution, it has consequently been practiced to receive the radio waves transmitted from a plurality of GPS satellites, and measure and analyze an integrated carrier phase and obtain the three-dimensional coordinate values of an unknown point with a high accuracy.
However, the radio waves transmitted from the GPS satellites are subject to various kinds of obstacles, and their receiving on the side of the receiving apparatus is sometimes interrupted instantly or continuously. Until the obstacles are removed, the radio waves to be transmitted from the GPS satellites cannot be received. Consequently, the amount of changes of the integrated carrier phase during this interruption cannot be known. This kind of dropping or lacking of measured data due to receiving obstacles is called a "cycle slip" and is regarded to be a serious problem in the positioning operation using the GPS surveying system.
By the way, this kind of receiving obstacles rarely occurs through a fault of the GPS satellites or the receiving apparatuses themselves, but mostly occurs due to a receiving environment. As examples of this kind of obstacles, the following can be listed.
1) Interruption by obstacles in the form of objects on the ground such as branches of trees, power cables, telephone cables, etc. while the GPS satellites are behind the shades thereof. PA1 2) Adhesion of snow and/or ice on an antenna. PA1 3) Out of tuning of the phase synchronization loop of the receiving apparatus due to noise obstacles of excessively strong pulses from electric equipment such as an electric resistance welding machine, rails of electric trains, or the like. PA1 4) Malfunctioning of the phase synchronization loop due to jamming radio wave disturbance which is continuous to a certain degree such, for example, as radar radio waves. PA1 5) Obstacles due to fading and shades through interference by reflecting radio waves against helicopters, airplanes, or the like. PA1 6) Obstacles by birds such as flying in stocks around an antenna or perching of birds on the antenna.
Among these obstacles, the cycle slip due to the above-described item 1) is likely to occur when GPS satellites of smaller angle of elevation are observed. They can, however, be prevented by paying attention to the location of placing the antenna of the receiving apparatus. The cycle slip due to the above-described item 2) can also be prevented to a certain degree by taking the necessary measures considering the location of placing the antenna or the meteorological conditions.
The obstacles due to the remaining items are, however, generally hard both to anticipate and to prevent, and the occurrence of interruption of receiving the radio waves cannot be avoided. Once the interruption of receiving has occurred and consequently the measured values contain a cycle slip, it becomes necessary, apart from the case where measurement is made again, to correct the phase amount of the carrier waves that changed during that period.
When the receiving apparatus detects the phase of the carrier waves, it can directly detect the values within the range of 0.degree. through 360.degree., but it cannot directly detect the amounts exceeding 360.degree.. Therefore, it is necessary to continuously detect the phase of the carrier waves and store the amount of changes beyond 360.degree. by, for example, advancing a counter each time 360.degree. is reached. If the phase is represented as the number of waves, 360.degree. will be "1" and the advance amount of the counter is a value representing the phase as it is. The integrated carrier phase .omega..sub.s based on a predetermined time can be represented by the value .omega..sub.m (0.ltoreq..omega..sub.m &lt;1) that is actually being detected at that instant and the value n on the counter at that instant as EQU .omega..sub.s =.omega..sub.m +n
The integrated carrier phase .omega..sub.s ' at a lapse of time .DELTA.t after the detection of the above-described .omega..sub.m can be represented by the value .omega..sub.m ' that is actually being detected at that instant and the advance amount .DELTA.n on the counter during the above-described .DELTA.t as EQU .omega..sub.s '=.omega..sub.m '+(n+.DELTA.n)
However, in case where the above-described period of time .DELTA.t happens to fall on the period of interruption of receiving in which the phase of the carrier waves cannot be detected, the counter cannot be advanced during the period of interruption of receiving. Consequently, the value of the above-described .omega..sub.m ' after ceasing of the interruption of receiving may be detected, but the advance amount of the above-described .DELTA.n cannot be obtained.
Here, since the advance amount .DELTA.n is an integer value, the integrated carrier phase that was lost during the interruption of receiving also becomes an integer value. It is in this point that major characteristics of the cycle slip of the GPS surveying lie.
In the conventional art, the following is practiced to cope with the above disadvantage. Namely, the measured values are recorded at every certain interval of time and, after a series of measuring procedures have been finished and at the time of "post processing" in which the three-dimensional coordinates of the unknown point are obtained, corrections are made for the cycle slip that occurred due to the interruption of receiving.
In the so-called kinematic surveying in which the integrated carrier phases are recorded while measuring them by sequentially moving the receiving apparatus among the unknown points, the correction of the cycle slip cannot be made after the measurement, should interruption of receiving occur during the moving operation. As a solution, in such a case, it is necessary to temporarily finish the measuring before the interruption of receiving occurs and to measure again by going back to the unknown point where the integrated carrier phase has been recorded.
The present invention has been made in view of these kind of disadvantages of the conventional art. The present invention has an object of providing a relative positioning GPS in which interruption of receiving of the radio waves, when it occurs, can be detected, and measured values are recorded after correcting the cycle slip, and the coordinate values of the three-dimensional coordinates of the unknown point can be obtained based on the recorded measured values.
Another object of the present invention is to provide an apparatus for carrying out the above-described relative positioning GPS.